The present invention relates to the field of encapsulation. It concerns more particularly an improved process for the preparation of an extruded product which encapsulates an active hydrophobic ingredient such as, for instance, a flavor or fragrance ingredient or composition. The process of the invention advantageously avoids any dehydration step before or after extrusion, thus improving the retention of such active ingredient during the entire process and in the final product.
In the flavor and perfume industry, extrusion is a widely used process for encapsulating active ingredients known to be volatile and labile. The flavor industry in particular is well fitted with a rich literature, notably patents, related to extrusion processes used for the preparation of encapsulated flavoring ingredients or compositions. This industry constantly seeks improvements for such processes and the products there-obtained, in terms of increase of the flavor retention, or of better control of the release of active ingredients from the finished products. In essence, the literature in the field of the invention discloses the encapsulation of flavor materials in glassy polymeric materials.
The concept of glass transition temperature (Tg) is well described in the literature in particular in relation to polymers. It represents the transition temperature of a molecular system from a rubbery (liquid) visco-elastic state to a glassy (solid) elastic state. As molecular systems are cooled below their Tg, their viscosity increases by several orders of magnitude within a more or less narrow temperature range. In the glassy state, i.e. at temperatures below Tg, molecules are frozen in a state of extremely low translation mobility.
It is recognized by many experts in the field that it is this low molecular mobility of glassy molecular systems which is used to stabilize actives in a solid dosage form. Implicit in much of the literature is the converse, namely that at temperatures above Tg, the encapsulation of flavor molecules will be ineffective and hence the importance of creating solid flavor capsules by formulating polymeric matrices characterized by Tg values higher than the ambient surrounding temperature.
The physical state of an encapsulating matrix can thus be expressed by the difference of T-Tg, where T is the temperature surrounding the system, i.e. the extrusion temperature when reference is made to the encapsulation process, and the ambient or storage temperature, namely a temperature typically comprised between 10 and 30° C. when reference is made to the storage of the final product, after the end of the extrusion process.
When T is equal to Tg, the surrounding temperature corresponds to the glass transition temperature of the system. For negative values of the difference of T-Tg, the system is in the glassy state and, the more negative is the difference, the lower the molecular mobility is within the system. Conversely, in the rubbery state, i.e. when the difference of T-Tg is positive, the more positive is the difference, the less viscous is the system. Thus by varying either the surrounding temperature T or the glass transition Tg of a given system, the latter can be either liquefied or solidified.
The glass transition temperature of a matrix can usually be adapted as desired by combining a thermoplastic polymer of appropriate molecular weight with a solvent that is able to lower the viscosity and thus the Tg of the neat polymer by plasticization. As an example, water can be used to plasticize the more hydrophilic polymers whereas less polar solvents are used to plasticize more hydrophobic polymers.
The difference of T-Tg evolves during the different steps of an encapsulation process and is representative of the changes in the physical state of the system.
In the encapsulation processes described in the prior art, a flavor is dispersed in a polymer, usually a carbohydrate matrix, which is maintained in a plasticized liquid state by properly selecting the processing temperature and the plasticizer concentration to fulfill the requirements for a positive difference of T-Tg. More particularly, the plasticizer concentration, in the prior art processes, is such that the difference of T-Tg is positive and greater than 100° C. to maintain the flavor phase dispersed homogeneously in the carbohydrate melt as it is extruded through a die. Therefore, the product exiting the die possesses a Tg which is too low (product in a liquid state) to produce a solid once the product has been cooled to storage temperature. As a consequence, all the extrusion processes described in the prior art comprise a drying step following the extrusion, which raises the final Tg of the extruded product above the ambient or room temperature, i.e. above a temperature varying between about 10 and 30° C., so that the difference of T-Tg is negative when T is the ambient temperature, thus providing a solid free flowing system. Free flowing extruded carbohydrate particles are thus only obtained once the sign of the difference of T-Tg has changed from being positive to becoming negative at storage temperature. These prior art processes present the problem of providing at the end of the extrusion a molten mass which is not sufficiently viscous to solidify at temperatures varying between 10 and 30° C., once shaped into the final desired product. Consequently, all these processes require, following the extrusion, an additional concentration, dehydration or drying step aimed at increasing the Tg of the extruded product above 10 to 30° C. WO 01/25414 discloses a typical example of this kind of process.
Now, a post-extrusion dehydrating or drying step presents obvious drawbacks such as mainly losing a part of the active ingredient encapsulated.
WO 01/17372, the content of which is hereby included by reference, has provided a solution to the mentioned problem of post-extrusion drying, and discloses a process wherein the extruded product possesses, at the end of the extrusion step, a glass transition temperature Tg sufficiently high to be shaped directly at the die end to yield a solid granular product once the extruded product has been cooled to storage temperature, without requiring a post-extrusion drying step. Practically this effect is obtained, as described in the examples of said application, by starting the process with a solid product, such as a dry blend or a spray-dried product. In other words, in the solution provided by the invention described in WO 01/17372, a starting emulsion is dried before being extruded. However, while indeed avoiding the problem linked to a post-extrusion drying step as mentioned before, the known process starts with a solid product and thus comprises a pre-extrusion drying step. Now, the latter presents the known drawbacks of any drying step, i.e. that it favors a loss of a part of active ingredient present in the starting emulsion, thus decreasing the retention of said ingredient during the entire process and in the final product.
Therefore, in order to optimize the fix of active ingredient in the final product, and to improve its retention during an extrusion process, a pre-drying step is also best avoided.
Now, up to now, no prior art document has ever disclosed or suggested a process able to entirely dispense with any dehydration steps, be it either before or after the extrusion of an emulsion, thus optimizing the retention of active ingredient from the starting emulsion to the final extruded granular solid.
The process of the invention provides such a solution to the problems created by drying steps in conventional extrusion encapsulation methods.